Extended Fuels Capability of Siemens’ SGT-400 DLE Combustion System
Brian M Igoe
Expert Proposal Manager (FEED)
Siemens Industrial Turbomachinery Ltd
Ruston House
PO Box 1, Waterside South Lincoln LN5
7FD
England
T: +44 1522 584418
brian.igoe@siemens.com
Andy Stocker
Product Manager SGT-400
Siemens Industrial Turbomachinery Ltd
Ruston House
PO Box 1, Waterside South Lincoln LN5
7FD
England
T: +44 1522 586895
andy.stocker@siemens.com
Abstract
To meet the growing demand to operate on gaseous fuels with little or no treatment, or use fuels derived
from a variety of waste conversion processes Siemens Energy has extended the fuels capability of its’
product range, especially the Dry Low Emissions combustion system. Fuels containing high levels of inert
species, nitrogen or carbon dioxide, lower the effective Wobbe Index of the fuels, thus needing increased
fuel mass flow to achieve the same energy content.
This paper presents the development process along with the results achieved to accommodate a wide
range of fuels. Discrete changes were required in the DLE burner hardware allowing fuel flows to be
achieved at similar supply pressures and combustor pressure drop as for standard fuels thus ensuring
combustion characteristics were not compromised.
Some applications are presented and discussed covering both on-shore and off-shore duty, including the
most recent application on the SGT-400 using a weak bio-gas derived from an ethanol production plant. A
gas only solution was applied, requiring careful management of the control parameters to achieve
acceptable starting and transient operation through to the application of load.
This programme demonstrates the ability of the Siemens DLE combustor to accommodate a wide range
of fuels.
1. Fuel Flexibility Introduction
In many parts of the world the use of gas turbines is a new or growing market and the availability of
premium pipeline quality gas fuels can be non-existent or very low. The ability to operate on poorer
quality fuels offers an alternative route in such evolving markets. The Siemens Dry Low Emissions (DLE)
combustion system, which is available across the small gas turbine products, has demonstrated
successful operation with over 17million hours of service experience in a wide variety of Oil & Gas and
Industrial Power Generation applications. The system meets stringent emission legislation limits over a
wide operating range and ambient conditions together with the ability to operate over the full load range.
This was therefore the combustion system of choice as the basis for developing an extended fuels
capability to cover a much wider range and type of fuels as shown in figure 1. These include wellhead gas
fuels with little or no treatment, or gas derived from industrial processes, which have qualities and heat
values below those of typical pipeline quality gas fuels.
Figure 1: Range of gaseous fuels
The SGT-300 was one of the first products, configured with DLE, to operate with gaseous fuels outside of
standard pipeline quality gas fuels and has since been followed with extended fuels capability on the
SGT-400.
Before embarking on the changes necessary in the combustion hardware and fuel delivery system to
accommodate the wide range of possible fuels, it is necessary to understand some of the key parameters
used to assess a fuel’s suitability.
2. Gas fuel Quality
Assessment of fuels proposed for use in a GT defines the critical features that need to be considered with
Wobbe Index, Calorific Value and Dew Point some of the key parameters applied.
Wobbe Index (WI) and Temperature Corrected Wobbe Index (TCWI) are two of many parameters
used to assess fuel and allows a direct comparison of different fuels to be made based on the
heat content. Wobbe Index (number) is the Net (lower) calorific value of the fuel divided by the
square root of the fuels specific gravity.
WobbeIndex
WI 0
CVv 0 / SG 0
Where CVv0
= net calorific value (MJ/m3) at standard conditions (288K, 1.013bara)
SG0
= specific gravity at standard conditions
The Temperature Corrected Wobbe Index (TCWI) applies a simple temperature correction. As
the supply temperature of a gaseous fuel is increased the TCWI falls. The range of acceptable
temperatures for use in a gas turbine varies from OEM to OEM and for Siemens Industrial
turbines the range is +2.5oC to +120oC. The lower figure prevents freezing in the vent of double
block and bleed part of the fuel system, whilst the upper limit, 120oC, is determined by some of
the components used in the fuel system.
=
×
_
_
Where W it_ref and Tt_ref refer to Wobbe Index and temperature at a known reference
condition
WIt is the revised Wobbe Index at the new, known, temperature Tt
TCWI becomes important when fuels contain some higher hydrocarbons, and, or water.
Assessing fuels with such additions to the composition results in determining the dew point of the
fuel. Gas Turbine OEMs apply a degree of superheat above the dew point to ensure liquid
condensate is eliminated. Siemens apply a 20oC superheat margin for the SGT-100 through to
SGT-400 product range.
3. Weak or Diluted Gas Fuels
Wellhead gas fuels often contain high levels of nitrogen and or carbon dioxide, which would require
specialist treatment to remove (or reduce to low levels) in order to create pipeline quality gas fuels.
Decomposition of waste, either from landfill sites or from controlled anaerobic digestion processes, will
produce high methane content gas fuels but with a WI significantly lower than for natural gas fuels.
Gaseous fuels diluted with inert species result in a reduced Wobbe Index, with both Nitrogen and Carbon
dioxide the major constituents influencing the fuels Wobbe Index, WI. Figure 2 shows the impact of inert
species nitrogen or carbon dioxide has on the fuel WI, with such weakened fuels considered during the
extended fuels development programmes. It should be noted for the same WI a higher proportion of
nitrogen can be accommodated.
50
UK Natural Gas
45
CO2
N2
Wobbe MJ/m
3
40
35
30
MCV development
25
20
15
10
5
LCV Burner
0
0
10
20
30
40
50
60
70
80
90
100
CO2 or N2 content of UK Natural Gas
Figure 2: Influence of inert diluents, CO2 or N2, on fuel Wobbe Index
Pipeline quality gas fuels contain mostly methane, with small quantities of ethane, and tend to fall into the
3
range 37 – 49MJ/m Wobbe Index. Weak, or Medium Calorific Value (MCV), fuels that include increasing
levels of inert species, such as nitrogen and carbon dioxide have a reduced WI, and can be as low as
17.5MJ/m3 (less than half the CV of natural gas), figure 3.
Figure 3: Wobbe Index Classification and SGT-300 Fuels Expansion Range
The lower WI gas requires an increased volume flow to achieve the same energy content as for typical
natural gas. The resulting increase in turbine power output could be seen as a benefit. However this
increased volume flow does increase the temperature of downstream turbine components. Therefore the
turbine entry temperature operating limit is reduced to maintain a similar or lower level of component
temperature, whilst providing a power output equivalent to that normally achieved with standard natural
gas.
4. Fuels Extension Programme
Besides design and analytical methods, Siemens gas turbine development facilities include a
comprehensive range of component test facilities, one of which is a high pressure combustion rig facility,
figure 4. As the Siemens DLE and Conventional combustion systems are of a Can-Annular design, this
permits testing of of a single combustion assembly at full operating conditions of the turbine model (ie full
engine pressure and temperature).
Figure 4: 4th generation DLE combustion test in the HP rig Facility Siemens Lincoln.
Supporting the high pressure combustion rig is a facility which can blend a range of fuels together with
inert species, thus providing the means to achieve a wide range of fuels likely to be encountered. This is
shown in figure 5 below.
Figure 5: Gas mixing facility arrangement – schematic and physical arrangement
Comprehensive testing across a wide range of fuels was completed, with detailed measurements made
including component temperatures, combustor exit profile (ensures gas temperature profile seen by
turbine blades is acceptable) and exhaust emission levels, particularly oxides of nitrogen (NOx) and
carbon monoxide (CO). Benchmarking of both existing and new burner designs were completed allowing
coverage over the Wobbe Fuel range from approximately 50MJ/m3 to approximately 15MJ/m3.
Siemens Industrial Gas Turbines configured with DLE combustion systems have operated on a wide
range of fuels. In recent years further extensions to fuel flexibility has been achieved (applying results
from development work described above) with many additional units now in commercial operation. In
order to verify and release an extended fuels capability, a full range of test methods have been used,
from the dedicated high pressure (HP) combustion rigs figure 4 through to part and full engine tests prior
to release. Necessary design changes are subject to rigorous design methods and tools to ensure the
final design achieves the lifing requirements. All development methods used follow the rigorous Siemens
Product Development Process (PDP) to ensure the changes and improvements comply with stringent
requirements and that risks are managed.
Take, for example, the case of the LNG plant, the “off-gas” can have both a high content of nitrogen and
also a high content of heavy hydrocarbons. Using the Wobbe Index parameter, as described earlier: as
the nitrogen content in the fuel increases, the Wobbe Index decreases such that at approximately 40 mol
% nitrogen the WI is halved. Fuel flow to the gas turbine increases accordingly, and for a standard
engine/combustion configuration this can be handled by increased pressure in the fuel-feeding system, or
by minor modifications to the burner fuel passage and injection geometry to maintain similar supply
pressure and pressure drop across the combustor hardware when compared to standard fuels, figure 6.
Gas Pressure upstream of burner (Pa)
2050000
With
no burner geometry
Standard
changes
Yadana increased fuel
supply
pressure required as
30-37MJ???
fuelTGCI
Wobbe Index decreases
2000000
1950000
1900000
1850000
1800000
1750000
15000
20000
25000
30000
35000
40000
45000
50000
Wobbe Index (kJ/m3)
Figure 6: Effect of burner pressure as Wobbe Index falls – indicates when change in geometry is required
5. Fuel Flexibility Applications
One of the first applications extending the capability of the DLE hardware beyond the standard range of
gas fuels as shown in figure 3 above was for an off-shore duty, for 3 off * SGT-300 power generation
sets for the platform, shown in figure 7. In addition to a gas containing both CO2 and N2 high levels of
Hydrogen Sulphide, H2S was present. The SGT-300 configured with standard DLE (and turbine)
hardware was able to operate with no concerns on this gas fuel. High Pressure rig testing of different
fuels at true engine temperatures and pressures in a single combustor, permitted the release of the
standard DLE combustion hardware over a much wider range of fuel compositions, including the fuel for
this application which contained 17-20 mol% CO2 and N2 combined and a TCWI circa 32-34MJ/m 3.
Figure 7: 3 * SGT-300 operates on fixed platform Mediterranean Sea
The success of this application provided the design basis for the SGT-300 gas turbine at University of
New Hampshire, where the ability to operate on a gas fuel with a lower WI was fully utilised, figure 8. The
fuel is a processed landfill gas (PLG), typically with a WI of circa 28-34MJ/m3. When assessed a minimum
set point for operation was agreed, 32MJ/m3, thus when the WI dropped below 32MJ/m3, or was
insufficient quantity for the GT duty, pipeline quality gas was blended as required, figure 9. Further details
are provided later.
Figure 8: SGT-300 at University of New Hampshire, operates with a Processed LandFill Gas PLG)
Figure 9: SGT-300 – UNH process screen shot (courtesy UNH)
Subsequent benchmarking of the SGT-300 DLE burner hardware, including further HP rig testing, has
3
confirmed 2 burner variants covering the Temperature Corrected Wobbe Index range from 17.5MJ/m
3
through to 49MJ/m . The lower limit for each burner range is dictated by supply pressure constraints as
well as pressure drop across the burner (maintains optimum combustion, ref figure 6 above). The
3
standard DLE combustion system is capable of operating to approximately 25MJ/m , with limiting factors
identified of supply pressure requirements and pressure drop across the burner. Increasing burner
delivery gas ejection hole size to maintain acceptable supply pressure and pressure drop has also been
3
evaluated, again in the combustion rig and a capability down towards 15MJ/m with commercial release
3
set at a minimum WI 17.5 MJ/m .
The same extensive programme was completed on the SGT-400 combustor with 3 burner derivatives
released to cover the wide WI fuel range 17.5MJ/m3 through to 49MJ/m3. Commercial opportunities have
been developed and experience gained at fuels circa 27MJ/m3 (offshore platform – well head gas);
34MJ/m3 (onshore - weak wellhead gas), and most recently an application at 22MJ/m3, (biogas from
Ethanol Industry waste).
6. CASE STUDIES
6.1. University of New Hampshire – SGT-300 in co-generation application
Initially sold as a dual fuel, natural gas and No2 distillate, the customer requested an operation with a
weak gaseous fuel to be investigated. Access to a landfill derived gas, LFG, was confirmed along with
a request to permit operation using the same DLE combustion configuration as supplied to the GT.
Investigation concluded it was not possible to operate on the raw LFG, but it would be possible to
operate on a gas fuel with a WI based on combustion testing, and commercial experience of the offshore application described earlier, ie down to approximately 30MJ/m3. To achieve this, the LFG had
to be processed to remove CO2 content, leaving the residue nitrogen content as the dominant inert
species. The gas fuel, processed landfill gas (PLG), typically with a WI range 28-34MJ/m 3, required a
minimum set point for operation, 32MJ/m3. When the WI dropped below 32MJ/m3, or there was
insufficient flow for the GT duty then pipeline quality gas was blended to achieve the minimum setpoint. This was seen as a significant technological advance offering genuine tri-fuel capability for this
industrial product, natural gas, processed landfill gas and No2 distillate liquid (diesel fuel) whilst still
meeting stringent local exhaust emissions regulations across the varying ambient and load
conditions.
6.2. Off-shore Application SGT-400 SE Asia
Applying DLE configurations to off-shore duty has been successfully achieved, with numerous
projects completed on both fixed and floating projects. One of the early projects for an off-shore duty
was described earlier. At a similar time frame a multiple application for SGT-400 was undertaken, for
a fixed off-shore platform in SE Asia (off-shore Myanmar), figure 10. Adopting the control product
development procedure, PDP, the high pressure combustion rig was fully used to assess the
necessary changes in combustion hardware to accommodate a weal wellhead fuel with high N2 and
CO2 content, with a resultant Temperature corrected Wobbe Index of circa 27MJ/m3. The burner
changes made allowed similar supply pressures as well as maintaining similar pressure drop across
the combustion system to be maintained commensurate with standard fuels, figure 6. SGT-400
proving tests were witnessed by the customer and included ignition and pull await tests during a full
package test with site equivalent gas provided in bottle form.
Figure 10: SGT-400 fixed platform duty off-shore Myanmar – includes fuel flexibility benefit
6.3. SGT- 400 power generation application in China using the bio-gas derived from the
production of Ethanol.
This example demonstrates the need to link all aspects of turbine capability and control to achieve a
satisfactory outcome without resorting to an alternative fuel.
An Ethanol processing plant produced a waste biogas containing high amounts of inert species,
nitrogen and carbon dioxide and this biogas was evaluated for the main fuel for an SGT-400 GT, rated
at 12.9MWe. A typical fuel composition and analysis of the biogas is shown in table 1 below.
Siemens extended fuels capability released on the SGT-400 covered fuels as low as 17.5MJ/m 3 Wobbe
Index (WI), and for this particular application the WI was determined at 21MJ/m3, at a required supply
temperature of >50OC. A gas only solution was required. Installation and commissioning was completed
during April and May 2013.
Species
Vol %
O2
0.21
N2
3.11
CO2
36.73
CH4
59.85
H2
0.02
H2S
0.08
Total
100%
Saturated at ambient conditions
Table 1: Biogas composition (2013)
Ethanol Production Plant
Product:
SGT-400
Location:
China
Operation on biogas:
Composition: CH4
60mol%;
CO2
37mol%
0
TCWI
21-22MJ/m3 @ ~55 C
Commissioned May 2013
Starting on biogas
No alternative fuel required
Figure 11: SGT-400 during commissioning (May 2013)- (Note: GT Exhaust is the shorter stack on left
side of photo.)
Start-up and turbine Running
Although the combustion rig testing confirmed ignition and load operation were satisfactory, it was
recognized that the transient operation and maintaining sufficient fuel flow (sufficient heat input to
sustain stable combustion) would need to be developed and optimized on the complete packaged unit.
Therefore, it was expected that numerous parameter adjustments would be required as part of the
commissioning phase.
Turbine Ignition
Attempts to light the turbine were made applying standard parameters, such as ignition speed of 3000
rpm. Using multi-light mapping tool (software coding), which allows fuel demand to be stepped in
increments for light up tests, it was not possible to light all 6 combustors
Lowering the ignition speed and ensuring gas was correctly conditioned for dew point control (achieved
by a degree of re-circulation), resulted in successful ignition with a speed of 2000rpm and 80% fuel flow
passing through pilot burner (80% pilot). Other parameter changes were introduced, such as the gas
generator purge speed (also reduced to 2000 rpm) with a corresponding increase in purge time.
Turbine Run Up To Full Speed
With a satisfactory ignition window achieved the next area to address was maintaining a stable
combustion and eliminating burner blowing out during acceleration from ignition to idle condition.
Changes to starter motor power and ramp rate were only partially successful, due to close proximity
with the compressor surge line. Resolution came with changes to the acceleration map (software code)
in order to maintain a near constant air fuel ratio for combustion. To compensate for the slow
acceleration of the gas generator more control parameter changes were necessary such as the link
between compressor discharge pressure and fuel flow. Idle conditions were now achievable, allowing
synchronizing and loading to be evaluated next.
Turbine Synchronizing and Load Application
Synchronizing was completed allowing load to be applied, leading to the next series of adjustment
iterations and optimization, initially in regard to flame stability within the burners. As with starting, the
standard fuel split maps were applied, as no experimental data from using contract fuel composition
was available. Load was applied steadily in 1Mw increments until 8Mw was reached. At this point
combustion dynamics (band 1 - indicator of an unstable flame) increased, requiring adjustment to the
running split map (increasing pilot fuel schedule). Continuing to increase load again resulted in band 1
combustion dynamics re-appearing. Evaluating the data recorded during these runs allowed Siemens
Combustion Engineering to recommend the necessary changes in fuel scheduling, including
modifications to some parameter absolute maximum limits from those used for natural gas running.
With these adjustments in place it was possible to load the turbine to maximum capacity.
It is clear when the GT is operating on very weak fuels, significant changes to the various control
parameters are required to achieve a satisfactory ignition window and loading regime.
6.4. SE Asia SGT-400 industrial GT utilizing multiple fuel streams of different calorific value
2 gas streams were identified for this application, the first containing inert content and the second a
normal pipeline quality gas fuel. Initial assessment confirmed both fuels could be used, but with
differing burner configuration (standard and one of the two MCV types). This was not practical as
both fuels needed to be accommodated in the same engine hardware configuration, to be able to
responding to a potential upset condition when the main, weak, fuel supply could be suspended.
The burner variant covering 30 – 37MJ/m3 fuel range was benchmarked, and confirmed to operate on
the wider range defined for this project. Rig and core engine testing were planned and completed
prior to contract release. A full package test, including customer witness, was also completed to
demonstrate the operation on the full range of fuels by introducing varying amounts of nitrogen in the
gas supply to the turbine, figure 12. The series of tests also included the turbine response to changes
in fuel calorific value, where a rate of change of 10%/minute with margin was successfully validated.
Figure 12: Nitrogen generator feeding N2 to gas supply for full engine test
Conclusion
An extended gas fuels capability range has been successfully developed and implemented for the
Siemens DLE Combustion system on the SGT-300 and SGT-400, to address the growing gas turbine
market in areas where the availability of premium fuels is low or non-existent. The service experience
with these fuels has been achieved on a number of applications in both on-shore and off-shore duties.
The most recent application in China utilizes the weakest fuel ever used in an SGT-400 of circa WI
21MJ/m3 and has now been successfully commissioned as a gas only installation.
The expanded Dry Low Emissions combustion capability into diluted gas fuels has been subject to
rigorous R&D programmes completed under the Siemens Product Development Process. Besides
fundamental design and analytical work, comprehensive combustion rig testing along with both core
engine and full packaged unit testing has been completed allowing the SGT-300 and SGT-400 increased
capability products to be released for commercial operation.
The Siemens DLE combustion systems has been developed to operate on a wide range of gases,
including gas fuels available in an LNG Liquefaction plant, from minimally processed weak wellhead type
fuels, even to biogas derived from landfill or anaerobic digestion processes.